In recent years, the projection-display industry has undergone a period of explosive growth. Until a few years age, such projection display systems were predominantly based on either cathode-ray tube (CRT) or active-matrix liquid crystal display (LCD) technology. However, all of these traditional display systems suffer from limitations that compromise their performance or the spectrum of their applicability. LCD-based and CRT-based systems are limited in their ability to support high-brightness applications, and they suffer from uniformity and stability problem in large-scale applications.
In 1996, Texas Instrument (TI) introduced to the market a novel microelectro-mechanical system (MEMS) based, high performance projection technology that addresses these issues. This technology, called Digital light Processing (DLP), is based on the Digital Micromirror Device (DMD), a light switch that uses electrostatically controlled MEMS mirror structures to modulate light digitally, producing stable, high-quality imagery on screen. FIG. 1A shows a conventional substrate 100 having a plurality of MEMS mirrors 102 thereon.
MEMS devices such as mirrors, electric motors, springs cantilevered devices, and mechanical switches and oscillators can be formed on the same substrate along with electronic circuits. These tiny mechanical devices have movable elements. A movable element such as a mirror is patterned of material deposited onto a sacrificial layer. The sacrificial layer is then removed by selective isotropic etching in a release process which undercuts the mirror, freeing it from the substrate. Cantilevered devices such as mirrors, mechanical switches, tuning forks or other oscillators, and leaf springs, are similarly formed partially over a sacrificial layer, with an anchored portion connected to a subjacent structure.
The DLP system is a fast reflective digital light switch, which combines image processing, memory, a light source, and optics. It is monolithically fabricated by a complementary metal oxide semiconductor (CMOS) process over a normal CMOS integrated circuit (IC).
The RSM (Reflectivity Stealth Mirror) is a new development product of the DLP systems. It operates using accepted digital video techniques and transmits to the eye a burst of digital light pluses that the eye interprets as a color image. Each light switch has an aluminum mirror that can reflect light in one direction depending on the state of the underlying memory cell. FIG. 1D shows an RSM system, including the transparent substrate 100 with MEMS mirrors 102. The transparent substrate 100 is mounted above the CMOS substrate 110, with CMOS circuitry 112 thereon, and spacers between the substrates 100 and 110.
RSM fabrication processes have been developed by Taiwan Semiconductor Manufacturing Company since 2001. The process can be separated into two parts, including MEMS (Aluminum mirror) processing and operational CMOS IC processing. The MEMS process uses a glass wafer for optical applications.
The DLP light switch is manufactured by a CMOS-like process, using a transparent substrate 100, such as a glass wafer, to transmit light. However, use of a transparent substrate creates a transfer problem in the IC manufacturing process. Almost all currently available machines need to orient the wafer to control transfer of the wafer, and need to sense the wafer using an infrared detector. So it is common to apply a Ti/OX backside coating 104 on the back side 100a of the transparent substrate 100 (opposite the side on which the MEMS 102 are formed), as shown in FIG. 1B, to solve this problem.
However, the MEMS fabrication process still has several serious issues. First, although the Ti/OX coating 104 on the backside can solve the transfer problem, an additional process is required to remove the Ti/OX. This includes photo coating, backside etching, dry photoresist striping, and wet post clean. This places a large load on the backside etching capacity of the equipment, increasing cycle time and wastes capacity. In some cases, with Ti/OX removal included in the front end of line (FEOL) processing, preventive maintenance must be performed after processing 24 pieces due to etch rate decay and contamination. This is a burden for production.
Further, the back side of the wafer may be damaged (as shown by damaged surface 100b in FIGS. 1C and 1D) during the etching process. In some configurations, the wafer 100 is bigger then the pedestal (not shown) on which the wafer is mounted during etching. Thus, an outer annular region of the backside of the wafer 100 is exposed to etching gas during etching. In this situation, damage to the wafer backside 100b is hard to avoid.
Damage to the backside 100b of the wafer 100 causes transfer errors in the procedures that follow the Ti/OX removal, and may result in the need to scrap the wafer in the manufacturing line (e.g., if light transmission efficiency is affected).
The other disadvantage of the above mentioned process is that the backside Ti/OX coating 104 needs to be removed during the FEOL fabrication process, so that the wafer again becomes transparent. However, to perform wafer acceptance test (WAT), the infrared sensor of the equipment needs to detect the wafer flat side before it can transfer the wafer to the WAT station. Because the infrared sensor cannot perform the detection after the removal of the Ti/OX layer, the (now transparent) wafer 100 cannot be transferred. Thus, WAT is not available for transparent substrates.
An improved process for transparent substrates with a MEMS thereon is desired.